The generally used batch chromatography is a separation process using the principle of adsorption mechanism and is suitable for high-purity separation and laboratory analysis. Thus, it is widely used for the separation and purification of high-purity biosynthetic compounds, fine chemicals, food additives and the like. However, this separation process using batch chromatography has problems in that it requires the use of a large amount of solvents and is difficult to separate components when a difference in adsorption between the components is small. Furthermore, it is unsuitable for large-scale separation and continuous separation.
In an attempt to solve these problems, a true moving bed (TMB) adsorptive separation process is used. This TMB process utilizes the countercurrent concept, which is effectively applied in heat exchangers or extraction processes. In this TMB process, when flow is introduced to a fixed bed countercurrently to a moving bed so that a solution of a mixture to be separated is introduced into a column, a component having a strong adsorption property to the fixed bed will flow out of the column by the fixed bed flow while a component having low adsorption will move along the moving bed.
The TMB process has an advantage in that it allows pure separation of two components if they can be separated only at both ends of their concentration distribution curve, even when there is no great difference in resolution between the two components. However, the TMB process has shortcomings in that it requires the use of a larger amount of filler than that of the existing fixed-type separation process and is very difficult to be performed in normal conditions due to the friction and release of the filler. To overcome these shortcomings, a simulated moving bed (SMB) adsorptive separation process has been developed in which a adsorbent, as a fixed bed, is filled into a separation column, and ports located along the separation column are moved at given time intervals so as to simulate the countercurrent movement of the fixed bed.
The SMB process has been developed to solve a problem in the fixed-bed flow of the TMB process and is applied for the purification of p-xylene, separation of ethylbenzene, and preparation of chiral compounds from a mixture of aromatic hydrocarbons, and the like. This process is typically disclosed in U.S. Pat. Nos. 4,326,092 and 5,382,747, assigned to UOP Inc.
In the separation of p-xylene by the prior SMB process, high-temperature and high-pressure conditions need to be maintained in order to maintain adsorption/desorption rates at suitable levels and to maintain fluid at a liquid phase. Due to such process characteristics, the rate of fluid flowing through each bed can greatly change suddenly in a moment, thus making the flow of the fluid unstable. When eddy current flow occurs due to the instability of fluid flow, adsorbent particles in the upper portion of each bed will be fluidized so that fine particles will occur due to the friction between the adsorbent particles. These fine particles are built up in the upper portion of each bed so as to cause pressure drop, and particularly when the fine particles are locally concentrated in the upper portion of each bed without uniform distribution, a channeling phenomenon will occur to cause a further increase in pressure drop. This increase in pressure drop makes the system going beyond its mechanical limit. Also, increased channeling will have a fatal adverse effect on the purity and recovery of p-xylene separated.
Thus, in order to solve the above-described problems occurring in the prior SMB adsorptive process, the present inventors formed an inert ball layer in the upper portion of each bed, i.e., on an adsorbent layer, so as to prevent the fluidization of adsorbent particles in the upper portion of each bed, thereby completing the present invention.